A phenomenon frequently occurs in small bore hypodermic syringes having plastic syringe barrels, and especially those with a total medication capacity of one cc or less, of air retention in medication aspirated into the syringe barrel. It is believed this phenomenon, at least in part occurs due to the relationship of the surface tension and viscosity of the medication, the wetting coefficient of the plastic syringe walls, and the small internal bore diameter. Such air retention is identifiable as a bubble or bubbles which may form at any point or points along the length of the medication column aspirated into the syringe barrel. Retained bubbles of air require identification and evacuation prior to administration of the medication for two important reasons. Firstly, it is not healthy to inject air into tissues or the bloodstream. Secondly and perhaps even more importantly, the retained air displaces medication in the syringe barrel so the volumetric reading at the plunger piston incorrectly identifies the actual volume of medication in the syringe barrel. This leads the person administering the medication to incorrectly assume the dosage is correct. This may be fatal or at the least, result in ineffective or incomplete treatment.
Persons with normal vision, even medical professionals, find it difficult to detect the presence of bubbles in small syringe bores. The problem is greatly amplified for those persons who have poor eyesight. For example, diabetics, often visually impaired, are frequently required to inject themselves daily with medication. An unnoticed air bubble embedded in the contents of the diabetic's syringe may have a serious effect by displacing a volume of critically needed medication. Yet the most typical syringe used by diabetics is the plastic, small bore, disposable syringe; the very syringe which most frequently manifests the air retention problem.
Not only is it difficult to detect bubbles in small bore plastic syringes, it is also difficult to determine when and if detected bubbles have been properly evacuated from the syringe bore. Once identified, difficulty is often encountered in moving the bubbles to the syringe end for evacuation. The most common method used is tapping the barrel while holding the syringe with the needle pointing upwardly. This is done until observation reveals that the bubbles have collected in position to make the evacuation possible by plunger pressure.
Air bubble retention is particularly troublesome when using colorless medications such as are often used in small bore plastic hypodermic syringes. The interface between air and the colorless medication is especially difficult to detect. This is due to similarities of color of the air, liquid and optical properties of the plastic barrel walls.
The above problem has been recognized in prior U.S. patents but adequate solutions have been lacking.
For example, U.S. Pat. No. 4,178,071 to Asbell discloses a magnifying cylinder for insulin syringes. This device is comprised of an optical sleeve including a cylindrical internal bore for receiving the barrel of a syringe. Its purpose is to magnify the contents of the syringe to indicate a preset fluid level for medication within the syringe barrel. It is indicated in the specification that coloration could be provided along regions of the sleeve adjacent a slit on the back side of the sleeve for more readily visualizing the liquid level confined within the body of the syringe. It is indicated that "since insulin is usually colorless, a darkened background is useful but not necessary." While this device is serviceable to indicate a consistent dosage amount and does have the effect of magnifying the contents of the hypodermic syringe, it is a separate item from the syringe itself and does not specify the precise nature of the darkened background area whereby air bubbles in the medication could be easily visually detected.
Another magnifying attachment is shown in U.S. Pat. No. 2,586,581 to Tschischeck. This device incorporates a magnifying lens that attaches by means of semicircular clips to the barrel of a hypodermic syringe. The device amplifies the calibration indicia along the length of the barrel and the bottom surface of the syringe plunger, which is used for indicating volume within the syringe by its alignment with the calibrations along the barrel. This device represents a partial solution to the problem indicated above but, like the Asbell device, is separate from the hypodermic syringe itself and does not offer a definite visual detection arrangement for amplifying contrast between liquid and air within the syringe barrel.
The above two references are the only prior references known to deal even indirectly with the problem of air retention within small bore hypodermic syringes.
U.S. Pat. No. 3,690,312 to Leibinsohn discloses a venous pressure manometric device with a level magnifying arrangement. The level indicator is attached to the tubular "sight glass." A pattern is situated along the length of the indicator behind the sight glass. The pattern is shown as a "checkerboard" pattern that appears to occupy the full length of the tube. But unlike the drawing, the amplification of the pattern would occur only in the transverse plane and would enable detection of bubbles only through a visual comparison of the transverse widths of the darkened spaces. Various areas of the checkerboard pattern are transversely amplified by portions of the tube in which liquid is present and are diminished along potions of the tube where air is present. Since the width dimension of the strip described in the Leibinshohn patent is apparently at least equal to the external diameter of the sight tube, the pattern along the strip will constantly appear to occupy the full diameter of the tube regardless of whether air or liquid is present therein. The gas and liquid interface must therefore be distinguished by the reader's ability to visually distinguish between magnified and reduced transverse portions of the patterns through the sight tube. This arrangement is appropriate for persons having normal visual acuity, but to the visually impaired, the pattern may visually dissolve into a solid strip. Furthermore, the checkerboard pattern could confuse reading of calibrations on the opposite side of the tube.
U.S. Pat. No. 2,303,154 to Armstrong is concerned with the problem of detecting the air-liquid interface at the surface of a liquid column held in a conventional transparent volumetric laboratory glass tube. The meniscus at the top of the liquid column is made easier to locate by provision of a roughened strip along the length of the tube opposite to the volumetric indices. The strip width is stated to be about one third of the width of the instrument. It is important to note that the drawings show that the strip is magnified through the tube where air is present and is more magnified, or appears still larger where liquid is present. It is also stated that the strip may either be situated on the outside or inside surfaces of the tube wall, yet the visual images shown are the same in either instance. This incorrectly identifies the image as magnified when the strip is applied to the outward surface of the tube. Armstrong therefore would have lead one to consider that the visual difference between the two magnified images at the liquid-air interface, particularly if viewed through a small bore plastic syringe, would be relatively insignificant. Furthermore Armstrong in his solution to the particular problem of identifying the meniscus at the top of a fluid column, gives no consideration to the problem addressed in this application, of observing retained air within a plastic small bore hypodermic syringe. This is evident from the fact that the volume is from the interface of an air column and a liquid meniscus as taught in Armstrong, but a highly visible alignment between the syringe piston bottom and volume indicia on the syringe barrel.
The problem of correctly identifying the volume of a liquid column in a large bore, transparent hypodermic syringe is approached in U.S. Pat. No. 2,888,015 to Hunt. The solution, addressed specifically to the volume detection problem was to provide a multi-colored strip along the clear barrel of the syringe. The length of syringe barrel occupied by each color was to be associated with a particular volume of aspirated medication. No discussion whatever is made with regard to the air retention problem indicated above, or that the multi colored strip could be modified to provide an adequate solution to the quite different problem of observing retained air in plastic small bore syringes.
The present invention unlike that of Hunt and Armstrong, or any other known reference is specifically addressed to a solution of the particular problem of improving the visibility of air retained within an aspirated volume of liquid in small diameter (one cc or less) plastic hypodermic syringes.
This is done by providing a narrow reflective solid strip of a single color along an outward side of the small diameter plastic syringe barrel. The relationship between the tube diameter, the optical properties of the small bore plastic tube walls, and the strip width is such that the tube will have the appearance of containing a fluid of the particular strip color where liquid spans the tube bore. In areas of the tube containing air, the strip will appear narrow with clear spaces to either side extending to the interface between the air and liquid. The strip may be integral with the tube so no attachments are required. Full, brightly colored areas within the tube contrast with relatively clear areas and a reduced, diffused image of the same color dramatically emphasize the interface between gas and liquid within the tube, even to the visually impaired. This solution has no effect on the manner in which volumetric measurements are discerned. Volume measurements are still read in the usual way, by relating the syringe piston to volumetric indicia on the syringe barrel.